The main objective of the proposed research project is to study the molecular mechanisms of a group of metabolic reactions which involve the enzymatic alkylation of the carbon atom at position 5 of pyrimidine precursors of nucleic acids and to explore the therapeutic potential of this information at the cellular level. The proposed project is part of a long-range research effort aimed at the elucidation of the various functions of sulfhydryl groups in biopolymers as a potential basis for rational chemotherapy. The research plan is based on the working hypothesis that the mechanisms of these and some related reactions involve the addition-elimination of an enzymic sulfhydryl group across the 5,6 double-bond of the pyrimidine nucleotide substrates. The problem is approached by studying the effects of specifically designed inhibitors on the kinetics of a number of enzymic reactions, as well as by studying the effects of selective modifications of the active site sulfhydryl groups on the catalytic properties of these enzymes. The study is extended to investigate the correlation between the effects of the inhibitors observed in the isolated enzyme systems and their effects on nucleic acid metabolism in intact, growing cells.